700R4 Overdrive Transmission Rebuild - Overachieving Overdrive

No matter how impressive the parts, paint, or power numbers, a Pontiac that can't move under its own power is a lawn ornament. Although having a reputation for being reasonably reliable with the stock engine output of 190 hp, we knew we were tempting fate with the stock 700-R4 trans in our '88 GTA by subjecting it to repeated dyno flogs at 300 rwhp. We hoped to delay a trans rebuild until we had some respectable dyno sheets, but recent testing on MD Automotives' Westminster, California, chassis dyno resulted in the trans spitting fluid from the breather; then Second, Third, and Fourth gears mysteriously vanished like intelligent conversation at a political rally. It was decision time. Should we beef up this transmission or seek other alternatives?

For a street-strip car, the 700-R4 has three distinct advantages over replacing it with a more robust Turbo 400 three-speed automatic, like many Third-Gen racers have done. First, it has a 3.06:1 First gear, thus a 700-R4-equipped vehicle with 3.55:1 rearend gearing accelerates like a Turbo-400 (2.48:1 First gear) car would with a 4.38:1 rearend. Second, the 700-R4's 0.70 (Overdrive) Fourth gear provides a cruise rpm equivalent to a Turbo 400 (1:1 Third gear) with 2.48:1 rearend gearing-the best of both worlds. Third, the 700-R4 has a lockup torque converter feature that locks the drive and driven portions of the converter with a clutch above 38 mph, completely eliminating slippage and the significant heating of the fluid from higher-stall-speed torque converters.

2/14A group of 700-R4 transmissions await shipment to customers.

We decided to retain the 700-R4, but seeing our Pontiac loaded onto a tow truck left us with the sinking feeling that a garden-variety rebuild wouldn't survive the planned dose of (hopefully) more power and yet more chassis dyno drubbings. Thus, we sought professional help.

Bowtie Overdrives' RecommendationsBowtie Overdrives (BTO) specializes in building high-performance hydraulic and electronically controlled GM overdrive transmissions. After hearing we were going to continue looking for more power above the current 450 lb-ft of engine torque output, and that we also intended to abuse the Bird on the dragstrip, BTO's general manager Chris Gardner suggested swapping in the company's Level 3 Extreme Duty trans ($1,595 outright, no core charge) for our broken 700-R4, along with a 2,600-stall-speed converter ($569, 2600 Extreme Duty) with built-in lockup clutch. This combination promised neck-snapping launches, solid shifts, and increased durability with minimal degradation of street driveability.

3/14This cutaway of the 700-R4 illustrates where the components are located.

The Autopsy And ReanimationTransmission removal was quick for the BTO boys, taking about 20 minutes. Upon disassembly, we found a number of issues that caused its demise. A primary telltale sign was the burnt, brownish fluid and its color-matched varnish cooked onto the transmission's internals. Gardner says overheating is the No. 1 reason for transmission failures. He advocates not only an external trans fluid cooler, but a temperature gauge as well. He warns that automatic transmission fluid (ATF) starts to degrade (burn) above 185 degrees F and loses its ability to lubricate and flow properly, thus dramatically reducing transmission life. He also states that virtually all the heat comes out of the torque converter, and the higher the converter stall speed, the more heat it creates before it locks up.

Our 700-R4's unwillingness to shift seemed to be due to a sticking governor, which was coated by burnt trans-fluid varnish. BTO's first priority is to sanitize or replace all internal parts in every transmission the company builds. Even the trans cooling lines and radiator heat exchanger are hooked up to a cleaning/purging machine.

Increasing Line PressureBTO's second priority is to eliminate slippage and wear by revising the hydraulic system to increase line pressure quickly as power is applied, as well as eliminate high-rpm pressure drop-off. To that end, the pump is completely blueprinted, beginning with a new, hardened stator. Both sides of the pump halves are then CNC-machined flat, indicated from the new stator. This ensures the stator is perpendicular and creates precision mating of the pump halves to eliminate internal pressure leakage.

Special hardened-steel pump-guide rings are installed to prevent pressure drop above 5,500 rpm and allow the driver to control the shift points manually without ill effects. Then BTO installs a new high-performance, 10-vane pump rotor, and upgrades to oversized 0.500 low-boost and 0.290 reverse-boost valves.

Teflon-coated front bushings are used from the later 4L60E, and new heavy-duty stator support bushings are also installed. The pump-regulator valve is modified to increase flow and is installed with a stiffer pressure-regulator spring. Finally, the pressure curve is recalibrated for quicker response off idle, which reduces slip with high-torque engines.

Decreasing Internal SlippageFriction and durability issues within the 700-R4 are tackled next. The 2-4 gear band assembly is upgraded to a wider 3/8-inch Kevlar band with a reinforced anchor to help eliminate slippage on shifts under full power.

5/14A hardened stator is pressed into the pump for improved durability; one can be seen here, both installed and sitting next to the pump for illustrative purposes. The double-lip premium front seal is also shown. A pump-regulator valve (already installed) is modified to increase flow and was installed with a stiffer pressure-regulator spring. The cover for the valve is visible here.

Next are a heavy-duty, 29-element input sprag clutch to withstand the solid shifts and a modified governor to raise automatic WOT shift points to the 5,200 to 5,600 rpm range (dependent on combination). The 3-4-gear clutch pack is upgraded to nine friction and nine steel plates, along with the BTO-designed special spring kit for quicker 3-4-gear clutch release. Premium-quality Borg Warner high-performance friction plates are installed throughout all clutch packs.

BTO then installs its custom-machined, oversized servo and cover assembly. The reverse drum is measured to ensure concentricity and trued if necessary; it's then installed with extra-width bushings to prevent rocking and absorb the greater loads of more powerful engines. A hardened input shaft and reinforced stock input drum and sun shell are then installed for optimum strength for engines over 400 lb-ft of torque (Level 3 and 4), and a 30 percent larger Fourth-gear servo piston is added.

RecalibrationThe part-throttle, throttle-valve accumulator circuits in the valvebody, and the spacer plate passages are then recalibrated for improved shift timing, firmness, and more precise pressure regulation. BTO considers these mods proprietary, so we don't have specifics on them.

The line-bias system is also modified. All these factors must work in concert with the TV cable actuation, which controls line pressure to make the transmission shift at the right time and with the right firmness for each throttle position and rpm range. BTO also upgrades front and rear seals to an aftermarket double-lip style, using premium one-piece seals throughout to minimize internal and external leaks.

Deflect The HeatOnce assembly is complete, the new torque converter is prefilled with ATF before it's plugged into the front of our sparkling-clean unit. The whole assembly is raised under the GTA, bolted back into place, topped up with ATF, and checked for leaks.

Quality ControlAn interesting feature of BTO's transmission quality control is that the company installs the transmissions and road tests them in a specially equipped '73 Chevy Stepside pickup behind a GM Performance Parts Ram Jet 350 to evaluate function and feel before they're shipped. This road-testing policy has resulted in warranty returns of less than 1 percent, saving everyone a lot of frustration.

9/14The 3-4-gear clutch pack is upgraded to nine friction and nine steel plates, (shown installed) along with the BTO-designed special spring kit (not shown), which results in a quicker 3-4 gear-clutch release.

Another BTO feature is that the one-year limited warranty doesn't start until the transmission is installed, and the pressure and temperature are checked and called in by the customer. Thus, if you're working on a project that becomes far more long-term than you intended (don't they all?), you won't burn up your transmission warranty while it sits on the floor of your garage.

Drive ReportOut on the road with the 2,600-stall converter, aggressive escapades with the throttle in First gear result in sideways, tire-boiling launches. Although this isn't the quickest way to the next stoplight, it does result in a toothy grin. It's less of a problem hitting Second gear; although the tires chirp hard, the GTA stays straight and hooks up again quickly. The Third gear shift gives a less dramatic chirp. By the time we approach Fourth, we're starting to wonder if the stock brakes are less adequate than we recall.

Around town, the 2,600-stall converter requires a tad more throttle and about 1,900 rpm to get the car moving from a standstill. It slips a bit more than the stocker when cruising normally until it locks up, and also softens the shifts at part throttle. Torque converter lockup is much more noticeable now: Engine rpm drops substantially-like going into Fifth gear-and a slip-free connection gives the Bird the feel of a manual transmission until tapping the brakes unlocks it.

10/14Higher horsepower applications often split the input drum around the input shaft splines. BTO makes a CNC-machined, steel brace (in hand, left) to press into the sun shell (right) to prevent this on Level 2 and up transmission builds.

Despite the same type of transmission with a higher-stall converter, we were surprised to find that on a 100-mile highway road trip, we actually showed a gain of about 1.6 mpg over the old iteration. This is probably the result of less internal drag from new bearings/bushings and, of course, the advantage of proper operation. The temperature gauge does run hot when we've been stuck in traffic on a 95-degree day, but it cools down quickly when we get moving and otherwise runs well within BTO's approved temperature range.

All in all, we learned a great deal about automatic transmissions and what makes them tick. Most importantly, we got our GTA back under its own power. We now have a great deal of confidence that whatever twist of fate puts our Pontiac on a tow truck again, it won't likely include the transmission as long as we maintain fresh fluid and keep the temperatures down.

Lockup For Vintage VehiclesBTO will also install an automatic Fourth gear and manual-converter clutchlockup system in custom-order applications at no charge. Although we didn't need this system, it allows for automatic Fourth-gear torque-converter lockup older vehicles without a computer, requiring the optional BTO wiring harness (PN TCCRELAY700; $35) that uses the brakelight switch to unlock the converter. This system also allows the use of an optional dash-mounted manual switch to lock/ unlock the converter, which is useful for locking the converter in lower gears if the owner is towing uphill or just going for maximum gas mileage.